Vehicle transmission systems, cooling systems, and braking systems may employ clutches or like devices to selectively transmit rotational forces from a drive source to an output member. For example, some cooling systems employ fan clutch devices that control the output rotation of engine cooling fans. Such a fan clutch device may be driven by a drive pulley that rotates in response to the vehicle engine.
In general, a clutch device can be operated to engage (or disengage) opposing clutch surfaces, which rotationally interconnect (or rotationally disconnect) the drive pulley and the output member. In an example related to fan clutches, when the clutch is shifted to the engaged position, friction surfaces engage and the output member (carrying fan blades) is driven to rotate along with the drive pulley. Over time, the friction surface may become worn, requiring replacement.
When a clutch is in a new condition, a gap of a predetermined size may exist between the clutch surfaces when they are in their disengaged position. To engage, the clutch surfaces may require a displacement corresponding to the size of the gap. Over time, repeated engagement and disengagement motions by the clutch surfaces may lead to wear. Such wear can be a normal and even a desirable aspect of the operation of a friction clutch device. For example, a wear member may be used where it is preferable to minimize wear on other components for reasons such as cost or complexity of replacement. On the other hand, the amount of wear that can be tolerated in a clutch system may also depend on considerations such as safety and available clearance space. For example, it may be important to avoid interference between parts of the clutch mechanism or movements outside the intended range of motion of the clutch mechanism. Accordingly, it may be desirable to have means for accommodating wear in the clutch surfaces of a clutch device, while also preventing the clutch from disengaging (or alternatively, preventing the clutch from engaging) when an unacceptable degree of wear has accrued.
In some cases, the fan clutch in a vehicle may become inoperable due to wearing of the friction surfaces. For example, in some conventional fan clutch devices, a friction clutch ring may be unable to engage an opposing friction surface when the friction clutch ring is worn below a threshold thickness. Accordingly, the fan clutch device may be unable to force engagement of the friction surfaces and the fan blades are not driven to rotate (e.g., no cooling airflow is provided). Due to the lack of cooling airflow, the vehicle's engine may overheat or otherwise become highly inefficient.
These conventional fan clutch devices are typically supplied with a set of “come home” bolts for separate storage by the vehicle operator, e.g., placement in the vehicle cabin for use at a much later time. Thus, after a period of years when the friction ring of the fan clutch device is worn down below a threshold level so that the friction surfaces are no longer able to engage, the vehicle operator must attempt to locate the “come home” bolts that were stored years earlier. If the “come home” bolts are located, the vehicle operator must then attempt to install the bolts to the fan clutch device in the engine compartment. In general, the “come home” bolts are installed onto the outer periphery of the clutch device so as to temporarily bolt the output member to the input member, thereby causing permanent rotation of the fan blades until the vehicle can be driven to a repair facility. For vehicles such as large semi-trucks or buses, the installation of the “come home” bolts may occur on the side of the road if the fan clutch fails during a long journey.